In the mobile communication network, communication quality is generally monitored based on an indicator (quality indicator) of the network statistical quality acquired from a communication device. For example, the quality indicator includes a call setup success rate, which represents the number of calls successfully set up with respect to the number of requests for setting up a call which occurs when a mobile terminal makes or receives a call, a handover success rate, which represents the number of handover operations successfully performed with respect to the number of requests for handover, and the like. Each mobile carrier uses a combination of the indicators. A network administrator checks the quality indicators and the output of a failure detection mechanism incorporated into the hardware or software of each communication device, thereby making it possible to carry out comprehensive quality and failure management. The general method to detect abnormalities of a network from the quality indicators is to compare the quality indicators with predetermined reference values which are preset for respective quality indicators. One of the conventional methods to set the reference values for respective quality indicators is to use a statistical average value of the past quality indicators (See PTL 1). That is, a statistical average value of the quality indicators during a predetermined period of time in the past when it was assumed that a communication network was normal is regarded as a reference value, the difference between the reference value and the actually measured value is calculated, and it is determined that abnormalities have occurred when the difference exceeds a predetermined threshold.